Prior art wheel speed sensors for vehicles have generally fallen into two broad categories, those employing either active or passive sensors. Active sensors require a power signal to be fed to a sensor component, whereas passive sensors need no outside power and usually consist of a coil surrounding a magnet material which is positioned in close proximity to a circular shaped element having a plurality of teeth which rotates with the wheel hub. The coil and magnet are mounted to a stationary part of the vehicle or axle assembly and the relative motion of the teeth in the rotor passing over a magnetized pole piece induces a change in reluctance which can be detected in an electronics package which is connected to the coil. A device of this type is described in U.S. Pat. No. 4,986,605, the disclosure which is expressly incorporated herein by reference, where a pulser and sensor unit are located inside the wheel hub mounted on the axle spindle in close proximity to a rotor connected to the wheel hub having a series of rectangular teeth.
Another type of passive wheel speed sensor is described in U.S. Pat. No. 3,887,046 and describes a sensor having a stator and a rotor where the stator is mounted on an axle and includes an electrical coil of wire surrounding the axle where the coil sets in a metal carrier having pole pieces and a plurality of magnets angularly disposed where the pole pieces are in close proximity to a multi-segmented rotor which is mounted to a hub cap which turns with the wheel hub.
A problem with these type of wheel speed sensors is that they require precision fabrication of the components so that, upon assembly, the limited clearance of the pole pieces to the rotor is maintained throughout the rotation of the wheel hub and hub cap assembly so that the signal due to the passing of the rotor teeth over the pole pieces is maximized.
Also, another problem with devices of this type is that the mounting of the stator piece is secured by the axle mount and is piloted on the outside diameter of the axle spindle which results in a large and complicated mounting system for the stator disc. If service of the stator is required, the prior art systems require that the spindle retention nut be disturbed which complicates the servicing process. Another problem occurs upon reassembly where the clearance between the rotor and the stator pole pieces is difficult to set and poor sensor performance is the potential result of that difficulty.